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Approximately 45% of the photosynthetically fixed carbon on Earth occurs in the oceans in phytoplankton, which account for less than 1% of the world's photosynthetic biomass. This amazing empirical observation implies a very high photosynthetic energy conversion efficiency, but how efficiently is the solar energy actually used? The photon energy budget of photosynthesis can be divided into three terms: the quantum yields of photochemistry, fluorescence, and heat. Measuring two of these three processes closes the energy budget. The development of ultrasensitive, seagoing chlorophyll variable fluorescence and picosecond fluorescence lifetime instruments has allowed independent closure on the first two terms. With this closure, we can understand how phytoplankton respond to nutrient supplies on timescales of hours to months and, over longer timescales, to changes in climate.

Variable fluorescence techniques are increasingly used to assess phytoplankton photosynthesis. All fluorescence techniques and models for photosynthetic electron transport rates (ETRs) are amplitude‐based and are subject to errors, especially when phytoplankton growth is nutrient‐limited. Here we develop a new, kinetic‐based approach to measure, directly and in absolute units, ETRs and to estimate growth rates in phytoplankton. We applied this approach to investigate the effects of nitrogen limitation on phytoplankton photophysiology and growth rates. Nutrient stress leads to a decrease in the quantum yield of photochemistry in Photosystem II (Fv/Fm); however, the relationship between Fv/Fmand growth rates is highly nonlinear, which makes it impossible to quantify the reduction in phytoplankton growth rates from Fv/Fmalone. In contrast, the decline in growth rates under nitrogen stress was proportional to the decrease in kinetic‐based photosynthetic rates. Our analysis suggests the kinetic fluorescence measurements markedly improve the accuracy of ETR measurements, as compared to classical amplitude‐based measurements. Fluorescence‐based methods for primary production rely on measurements of ETRs and then conversion to carbon fixation rates by using the electron yields of carbon fixation. The electron yields exhibit 10‐fold variability in natural phytoplankton communities and are strongly affected by nutrient limitation. Our results reveal that a decrease in the growth rates and the electron yields of carbon fixation is driven by, and can be quantified from, a decrease in photosynthetic turnover rates. We propose an algorithm to deduce the electron yields of carbon fixation, which greatly improve fluorescence‐based measurements of primary production and growth rates.

The West Antarctic Peninsula (WAP) is a highly productive polar ecosystem where phytoplankton dynamics are regulated by intense bottom‐up control from light and iron availability. Rapid climate change along the WAP is driving shifts in the mixed layer depth and iron availability. Elucidating the relative role of each of these controls and their interactions is crucial for understanding of how primary productivity will change in coming decades. Using a combination of ultra‐high‐resolution variable chlorophyll fluorescence together with fluorescence lifetime analyses on the 2017 Palmer Long Term Ecological Research cruise, we mapped the temporal and spatial variability in phytoplankton photophysiology across the WAP. Highest photosynthetic energy conversion efficiencies and lowest fluorescence quantum yields were observed in iron replete coastal regions. Photosynthetic energy conversion efficiencies decreased by ~ 60% with a proportional increase in quantum yields of thermal dissipation and fluorescence on the outer continental shelf and slope. The combined analysis of variable fluorescence and lifetimes revealed that, in addition to the decrease in the fraction of inactive reaction centers, up to 20% of light harvesting chlorophyll‐protein antenna complexes were energetically uncoupled from photosystem II reaction centers in iron‐limited phytoplankton. These biophysical signatures strongly suggest severe iron limitation of photosynthesis in the surface waters along the continental slope of the WAP.

Materials science has been informed by nonclassical pathways to crystallization, based on biological processes, about the fabrication of damage-tolerant composite materials. Various biomineralizing taxa, such as stony corals, deposit metastable, magnesium-rich, amorphous calcium carbonate nanoparticles that further assemble and transform into higher-order mineral structures. Here, we examine a similar process in abiogenic conditions using synthetic, amorphous calcium magnesium carbonate nanoparticles. Applying a combination of high-resolution imaging and in situsolid-state nuclear magnetic resonance spectroscopy, we reveal the underlying mechanism of the solid-state phase transformation of these amorphous nanoparticles into crystals under aqueous conditions. These amorphous nanoparticles are covered by a hydration shell of bound water molecules. Fast chemical exchanges occur: the hydrogens present within the nanoparticles exchange with the hydrogens from the surface-bound H2O molecules which, in turn, exchange with the hydrogens of the free H2O molecule of the surrounding aqueous medium. This cascade of chemical exchanges is associated with an enhanced mobility of the ions/molecules that compose the nanoparticles which, in turn, allow for their rearrangement into crystalline domains via solid-state transformation. Concurrently, the starting amorphous nanoparticles aggregate and form ordered mineral structures through crystal growth by particle attachment. Sphere-like aggregates and spindle-shaped structures were, respectively, formed from relatively high or low weights per volume of the same starting amorphous nanoparticles. These results offer promising prospects for exerting control over such a nonclassical pathway to crystallization to design mineral structures that could not be achieved through classical ion-by-ion growth.

Ambidoxin is a designed, minimal dodecapeptide consisting of alternating L and D amino acids that binds a 4Fe–4S cluster through ligand–metal interactions and an extensive network of second-shell hydrogen bonds. The peptide can withstand hundreds of oxidation–reduction cycles at room temperature. Ambidoxin suggests how simple, prebiotic peptides may have achieved robust redox catalysis on the early Earth.

The geosphere and biosphere coevolved and influenced Earth's biological and mineralogical diversity. Changing redox conditions influenced the availability of different transition metals, which are essential components in the active sites of oxidoreductases, proteins that catalyze electron transfer reactions across the tree of life. Despite its relatively low abundance in the environment, cobalt (Co) is a unique metal in biology due to its importance to a wide range of organisms as the metal center of vitamin B12(aka cobalamin, Cbl). Cbl is vital to multiple methyltransferase enzymes involved in energetically favorable metabolic pathways. It is unclear how Co availability is linked to mineral evolution and weathering processes. Here we examine important biological functions of Co, as well as chemical and geological factors that may have influenced the utilization of Co early in the evolution of life. Only 66 natural minerals are known to contain Co as an essential element. However, Co is incorporated as a minor element in abundant rock‐forming minerals, potentially representing a reliable source of Co as a trace element in marine systems due to weathering processes. We developed a mineral weathering model that indicates that dissolved Co was potentially more bioavailable in the Archean ocean under low S conditions than it is today. Mineral weathering, redox chemistry, Co complexation with nitrogen‐containing organics, and hydrothermal environments were crucial in the incorporation of Co in primitive metabolic pathways. These chemical and geological characteristics of Co can inform the biological utilization of other trace metals in early forms of life. Co is an important metal in various biological processes with potentially ancient origins, but a rare mineral forming elementOur mineral weathering model indicates that dissolved Co was potentially more bioavailable in the Archean ocean under low S conditionsMineral weathering, redox chemistry, and Co complexation were crucial in the incorporation of Co in primitive metabolic pathways

Understanding the cellular and molecular responses of stony corals to ocean acidification is key to predicting their ability to calcify under projected high CO2conditions. Of specific interest are the links between biomineralization proteins and the precipitation of new calcium carbonate (CaCO3), which potentially can provide a better understanding of the biomineralization process. We have assessed the effects of increased CO2on the calcification process in cell cultures of the stony coral, Stylophora pistillata, reared in nutrient‐enriched artificial seawater at four pCO2levels and two glucose concentrations. Dispersed S. pistillatacells grown at low (400 ppmV) and moderate (700 ppmV) pCO2re‐aggregate into proto‐polyps and precipitate CaCO3. When grown at pCO2levels of 1000 ppmV and 2000 ppmV, the cells up‐regulate genes for two highly acidic proteins as well as a carbonic anhydrase, but down‐regulate long term cadherin protein production and minimize proto‐polyp formation, and exhibit a significant decrease in measurable CaCO3precipitation. However, cell cultures precipitate CaCO3in all treatments, even at slightly undersaturated conditions (Ωaragonite< 0.95). Glucose addition does not influence either biomineralization gene expression or calcification rate. Measured δ11B of the mineral phase, as a proxy of the pH at the calcifying sites, is out of equilibrium with the ambient seawater medium surrounding the cells and proto‐polyps, suggesting pH is elevated in the micro‐environment of the precipitating mineral. Our results suggest that coral cells possess molecular mechanisms to help compensate for the effects of ocean acidification within the bounds projected in the coming century.

Life is based on energy gained by electron-transfer processes; these processes rely on oxidoreductase enzymes, which often contain transition metals in their structures. The availability of different metals and substrates has changed over the course of Earth's history as a result of secular changes in redox conditions, particularly global oxygenation. New metabolic pathways using different transition metals co-evolved alongside changing redox conditions. Sulfur reduction, sulfate reduction, methanogenesis and anoxygenic photosynthesis appeared between about 3.8 and 3.4 billion years ago. The oxidoreductases responsible for these metabolisms incorporated metals that were readily available in Archaean oceans, chiefly iron and iron–sulfur clusters. Oxygenic photosynthesis appeared between 3.2 and 2.5 billion years ago, as did methane oxidation, nitrogen fixation, nitrification and denitrification. These metabolisms rely on an expanded range of transition metals presumably made available by the build-up of molecular oxygen in soil crusts and marine microbial mats. The appropriation of copper in enzymes before the Great Oxidation Event is particularly important, as copper is key to nitrogen and methane cycling and was later incorporated into numerous aerobic metabolisms. We find that the diversity of metals used in oxidoreductases has increased through time, suggesting that surface redox potential and metal incorporation influenced the evolution of metabolism, biological electron transfer and microbial ecology.

Amundsen Sea polynyas are among the most productive, yet climate‐sensitive ecosystems in the Southern Ocean and host massive annual phytoplankton blooms. These blooms are believed to be controlled by iron fluxes from melting ice and icebergs and by intrusion of nutrient‐rich Circumpolar Deep Water, however the interplay between iron effects and other controls, such as light availability, has not yet been quantified. Here, we examine phytoplankton photophysiology in relation to Fe stress and physical forcing in two largest polynyas, Amundsen Sea Polynya (ASP) and Pine Island Polynya (PIP), using the combination of high‐resolution variable fluorescence measurements, fluorescence lifetime analysis, photosynthetic rates, and Fe‐enrichment incubations. These analyses revealed strong Fe stress in the ASP, whereas the PIP showed virtually no signatures of Fe limitation. In spite of enhanced iron availability in the PIP, chlorophyll biomass remained ∼ 30–50% lower than in the Fe‐stressed ASP. This apparent paradox would not have been observed if iron were the main control of phytoplankton bloom in the Amundsen Sea. Long‐term satellite‐based climatology records revealed that the ASP is exposed to significantly higher solar irradiance levels throughout the summer season, as compared to the PIP region, suggesting that light availability controls the magnitude of phytoplankton blooms in the Amundsen Sea. Our data suggests that higher Fe availability (e.g., due to higher melting rates of ice sheets) would not necessarily increase primary productivity in this region. Furthermore, stronger wind‐driven vertical mixing in expanding ice‐free areas may lead to reduction in light availability and productivity in the future.

We measured the light dependence of gross photosynthesis and oxygen uptake rates by membrane inlet mass spectrometry in two open ocean regions: the Amundsen Sea (Antarctica), dominated by Phaeocystis antarctica, and the North Atlantic, dominated by Emiliania huxleyi. In the North Atlantic, respiration was independent of irradiance and was higher than the gross photosynthetic rate at all irradiances. In contrast, in the Amundsen Sea, oxygen uptake processes were light dependent; dark respiration was one order of magnitude lower than the maximal gross photosynthetic rate, but the oxygen uptake rate increased by 10 fold at surface irradiances. Our results suggest the light dependence of oxygen uptake in Amundsen Sea has two sources: one is independent of photosynthesis, and is possibly associated with the photo‐reduction of O2mediated by dissolved organic matter; the second reflects the activity of an oxidase fueled in the light with photosynthetic electron flow. Our results highlight the importance of improving our understanding of oxygen consuming reactions in the euphotic zone.

By using a nondestructive, ultrasensitive, fluorescence kinetic technique, we measure in situ the photochemical energy conversion efficiency and electron transfer kinetics on the acceptor side of histidine‐tagged photosystem II core complexes tethered to gold surfaces. Atomic force microscopy images coupled with Rutherford backscattering spectroscopy measurements further allow us to assess the quality, number of layers, and surface density of the reaction center films. Based on these measurements, we calculate that the theoretical photoelectronic current density available for an ideal monolayer of core complexes is 43 μA cm−2at a photon flux density of 2000 μmol quanta m−2s−1between 365 and 750 nm. While this current density is approximately two orders of magnitude lower than the best organic photovoltaic cells (for an equivalent area), it provides an indication for future improvement strategies. The efficiency could be improved by increasing the optical cross section, by tuning the electron transfer physics between the core complexes and the metal surface, and by developing a multilayer structure, thereby making biomimetic photoelectron devices for hydrogen generation and chemical sensing more viable.

The NASA Astrobiology Roadmap provides guidance for research and technology development across the NASA enterprises that encompass the space, Earth, and biological sciences. The ongoing development of astrobiology roadmaps embodies the contributions of diverse scientists and technologists from government, universities, and private institutions. The Roadmap addresses three basic questions: How does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? Seven Science Goals outline the following key domains of investigation: understanding the nature and distribution of habitable environments in the universe, exploring for habitable environments and life in our own solar system, understanding the emergence of life, determining how early life on Earth interacted and evolved with its changing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth. For each of these goals, Science Objectives outline more specific high-priority efforts for the next 3-5 years. These 18 objectives are being integrated with NASA strategic planning.

Curiously, the benthic foraminifer, Nonionella stella, found in the upper 3 cm of sediments collected off California at a depth of ;600 m, retains chloroplasts. We examined the origin and physiological function of the organelles within the host cell. Transmission electron micrographs, fluorescence and absorption spectra, Western blots, and enzyme assays revealed that the chloroplasts were intact and retained functionality for up to 1 year after sample collection. 16S rDNA gene sequences established that the plastids were derived from diatoms closely related to Skeletonema costatumand Odontella sinensis. Western blots of three major chloroplast proteins (ribulose bisphosphate carboxylase oxygenase [RuBisCO], the D1 protein, and the fucoxanthin chlorophyll a protein complex) confirmed that the organelle retained both nuclear and chloroplast encoded proteins, which suggests that the turnover of the plastid machinery is extremely low. Moreover, the two carboxylating enzymes examined, RuBisCO and phosphoenol pyruvate carboxylase, retained catalytic activity. Three hypotheses regarding the function of sequestered chloroplasts in deep‐sea foraminifera were considered: (1) the organelles are photosynthetic under extremely low irradiance levels, (2) the organelles utilize exogenous substrates to generate an electrochemical gradient that permits chemoautrophy in the dark, and (3) they are used for the assimilation of inorganic nitrogen. Our results suggest that the chloroplasts are used to meet the nitrogen requirements of the host. Immunolocalization of the nuclear encoded protein, nitrate reductase, supports this hypothesis. This protein is widely distributed in photoautotrophs but is not encoded in eukaryotic protists such as foraminifera.

The physiological processes of natural phytoplankton mortality due to environmental stress (vs. that caused by sedimentation and predation) are poorly understood. Cell survival was examined in batch cultures of the diatom Thalassiosira weissflogiiand the chlorophyte Dunaliella tertiolectaduring deprivation of fixed nitrogen or light. Despite severe impairment of photosynthetic efficiencies, both species remained viable during 2 weeks of N starvation. Under N stress, a specific protease was induced in the diatom, overall activity of proteases doubled, and there was gradual, selective loss of certain proteins, especially ribulose 1,5‐bisphosphate carboxylase/oxygenase. Light‐deprived diatoms were virtually unaffected, but the chlorophyte underwent catastrophic cell death after about 6 d of darkness. Cell death coincided with a large increase in protease activity and the induction of a specific protease. Although we cannot completely rule out roles for viruses or bacteria in the losses of cells, the consistent timing, the unique response to stress, and the coincident expression of a specific protease strongly suggest that the process is a form of autocatalyzed cell death, such as apoptosis. While of uncertain adaptive significance, phytoplankton cell death may have implications for species succession and cycling of organic matter in aquatic ecosysterns.

Triton X-100, a detergent commonly used to solubilize higher plant thylakoid membranes, was found to be deleterious to Dunaliella LHC II. It disrupted the transfer of excitation energy from chlorophyll b to chlorophyll a. Based on analysis of pigments and immunoassays of LHC II apoproteins from sucrose density gradient fractions, Triton X-100 caused aggregation of the complex, but apparently did not remove chlorophyll b from the apoprotein. Following solubilization with Triton X-100 only CPI could be resolved by electrophoresis. In contrast, solubilization of Dunaliella thylakoids with octyl-ß-D-glucopyranoside preserved energy transfer from chlorophyll b to chlorophyll a. This detergent also effectively prevented aggregation on sucrose gradients and preserved CPI oligomers, as well as LHCP1 and LHCP3 on non-denaturing gels. Solubilization with Deriphat gave similar results. We propose that room temperature fluorescence excitation and emission spectroscopy be used in conjunction with other biophysical and biochemical probes to establish the effects of detergents on the integrity of light harvesting chlorophyll protein complexes. Methods used here may be applicable to other chlorophytes which prove refractory to protocols developed for higher plants.

The QBbinding site of the D1 reaction center protein, located within a stromal loop between transmembrane helices IV and V formed by residues Ile219to Leu272, is essential for photosynthetic electron transport through photosystem II (PSII). We have examined the function of the highly conserved Ala251D1 residue in this domain in chloroplast transformants ofChlamydomonas reinhardtiiand found that Arg, Asp, Gln, Glu, and His substitutions are nonphotosynthetic, whereas Cys, Ser, Pro, Gly, Ile, Val, and Leu substitutions show various alterations in D1 turnover, photosynthesis, and photoautotrophic growth. The latter mutations reduce the rate of QAto QBelectron transfer, but this is not necessarily rate-limiting for photoautotrophic growth. The Cys mutant divides and evolves O2at wild type rates, although it has slightly higher rates of D1 synthesis and turnover and reduced electron transfer between QAand QB. O2evolution, D1 synthesis, and accumulation in the Ser, Pro, and Gly mutants in high light is reduced, but photoautotrophic growth rate is not affected. In contrast, the Ile, Val, and Leu mutants are impaired in photoautotrophic growth and photosynthesis in both low and high light and have elevated rates of D1 synthesis and degradation, but D1 accumulation is normal. While rates of synthesis/degradation of the D1 protein are not necessarily correlated with alterations in specific parameters of PSII function in these mutants, bulkiness of the substituted amino acids is highly correlated with the dissociation constant for QBin the seven mutants examined. These observations imply that the Ala251residue plays a key role in D1 protein.

Photosynthetic organisms alter their photosynthetic pigment composition in response to changes in growth irradiance1–3. Photo-adaptation maximizes light-harvesting when photon flux densities are low, and minimizes photo-oxidative damage to the photosynthetic machinery at high light levels. In chlorophytes, the major light-harvesting antenna is light-harvesting complex II (LHC II), a family of proteins binding chlorophyll a (Chl a), Chl b, and carotenoids, and accounting for 40–60% of total cell chlorophyll4. LHC II is associated principally with photosystem II, with reversible phosphorylation of LHC II regulating short-term adjustments in energy distribution to photosystem I5. Previous studies on green algae and higher plants have emphasized the longer-term adaptive importance of the inverse relationship between growth irradiance and the proportion of total cellular chlorophyll associated with LHC II6–8. In higher plants the pigment composition of LHC II appears to be highly conserved, with Chl a/Chl b ratios between 1.0 and 1.2 (ref 7). In green algae, the pigment ratio of LHC II is more variable and values between 0.7 and 2.7 have been reported9–11. We report here that in the unicellular marine chlorophyte, Dunaliella tertiolecta, the ratio is actually variable. Photoadapta-tion to high irradiance involves changes in the average composition and behaviour of LHC II; specifically, the Chl b content per polypeptide is halved and the efficiency of excitation transfer from carotenoid to Chl a declines. The result is a novel mechanism for regulating the effective absorption cross-section of photosystem II.

The effects of PAR and UV radiation on PS II photochemistry were examined in natural phytoplankton communities from coastal waters off Rhode Island (USA) and the subtropical Pacific. The photochemical energy conversion efficiency, the functional absorption cross section and the kinetics of electron transfer on the acceptor side of PS II were derived from variable fluorescence parameters using both pump and probe and fast repetition rate techniques. In both environments, the natural phytoplankton communities displayed marked decreases in PS II photochemical energy conversion efficiency that were correlated with increased PAR. In the coastal waters, the changes in photochemical energy conversion efficiency were not statistically different for samples treated with supplementary UV-B radiation or screened to exclude ambient UV-B. Moreover, no significant light-dependent changes in the functional absorption cross section of PS II were observed. The rate of electron transfer between QA- and QB was, however, slightly reduced in photodamaged cells, indicative of damage on the acceptor side. In the subtropical Pacific, the decrease in photochemical energy conversion efficiency was significantly greater for samples exposed to natural levels of UV-A and/or UV-B compared with those exposed to PAR alone. The cells displayed large diurnal changes in the functional absorption cross section of PS II, indicative of non-photochemical quenching in the antenna. The changes in the functional absorption cross section were highly correlated with PAR but independent of UV radiation. The time course of changes in photochemical efficiency reveals that the photoinhibited reaction centers rapidly recover (within an hour or two) to their preillumination values. Thus, while we found definitive evidence for photoinhibition of PS II photochemistry in both coastal and open ocean phytoplankton communities, we did not find any effect of UV-B on the former, but a clear effect on the latter. The results of this study indicate that the effects of UV-B radiation on phytoplankton photosynthesis are as dependent on the radiative transfer properties of the water body and the mixing rate, as on the wavelength and energy distribution of the radiation and the absorption cross sections of the biophysical targets.

The diadinoxanthin cycle (DD-cycle) in chromophyte algae involves the interconversion of two carotenoids, diadinoxanthin (DD) and diatoxanthin (DT). We investigated the kinetics of light-induced DD-cycling in the marine diatom Phaeodactylum tricornutum and its role in dissipating excess excitation energy in PS II. Within 15 min following an increase in irradiance, DT increased and was accompanied by a stoichiometric decrease in DD. This reaction was completely blocked by dithiothreitol (DTT). A second, time-dependent, increase in DT was detected ~ 20 min after the light shift without a concomitant decrease in DD. DT accumulation from both processes was correlated with increases in non-photochemical quenching of chlorophyll fluorescence. Stern-Volmer analyses suggests that changes in non-photochemical quenching resulted from changes in thermal dissipation in the PS II antenna and in the reaction center. The increase in non-photochemical quenching was correlated with a small decrease in the effective absorption cross section of PS II. Model calculations suggest however that the changes in cross section are not sufficiently large to significantly reduce multiple excitation of the reaction center within the turnover time of steady-state photosynthetic electron transport at light saturation. In DTT poisoned cells, the change in non-photochemical quenching appears to result from energy dissipation in the reaction center and was associated with decreased photochemical efficiency. D1 protein degradation was slightly higher in samples poisoned with DTT than in control samples. These results suggest that while DD-cycling may dynamically alter the photosynthesis-irradiance response curve, it offers limited protection against photodamage of PS II reaction centers at irradiance levels sufficient to saturate steady-state photosynthesis.

The oxygen flash yield (YO2) and photochemical yield of PS II (FPS II) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, YO2 and FPS II followed each other, suggesting a close coupling between the oxidation of water and QA reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432–443). Following a light-to-dark transition, however, the relationship between QA reduction and the fraction of PS II reaction centers capable of evolving O2 became temporarily uncoupled. FPS II recovered to the preillumination levels within 5–10 s, while the YO2 required up to 60 s to recover under aerobic conditions. The recovery of YO2 was independent of the redox state of QA, but was accompanied by a 30% increase in the functional absorption cross-section of PS II (sPS II). The hysteresis between YO2 and the reduction of QA during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.

Coral skeletons are materials composed of inorganic aragonitic fibres and organic molecules including proteins, sugars and lipids that are highly organized to form a solid biomaterial upon which the animals live. The skeleton contains tens of proteins, all of which are encoded in the animal genome and secreted during the biomineralization process. While recent advances are revealing the functions and evolutionary history of some of these proteins, how they are spatially arranged in the skeleton is unknown. Using a combination of chemical cross-linking and high-resolution tandem mass spectrometry, we identify, for the first time, the spatial interactions of the proteins embedded within the skeleton of the stony coral Stylophora pistillata. Our subsequent network analysis revealed that several coral acid-rich proteins are invariably associated with carbonic anhydrase(s), alpha-collagen, cadherins and other calcium-binding proteins. These spatial arrangements clearly show that protein–protein interactions in coral skeletons are highly coordinated and are key to understanding the formation and persistence of coral skeletons through time.

The effect of various nitrogen sources on the synthesis and activity of nitrogenase was studied in the marine, non-heterocystous cyanobacterium Trichodesmium sp. NIBB1067 grown under defined culture conditions. Cells grown with N2 as the sole inorganic nitrogen source showed light-dependent nitrogenase activity (acetylene reduction). Nitrogenase activity in cells grown on N2 was not suppressed after 7 h incubation with 2 mM NaNO3 or 0.02 mM NH4Cl. However, after 3 h of exposure to 0.5 mM of urea, nitrogenase was inactivated. Cells grown in medium containing 2 mM NaNO3, 0.5 mM urea or 0.02 mM NH4Cl completely lacked the ability to reduce acetylene. Western immunoblots tested with polyclonal antisera against the Fe-protein and the Mo-Fe protein, revealed the following: (1) both the Fe-protein and the Mo-Fe protein were synthesized in cells grown with N2 as well as in cells grown with NaNO3 or low concentration of NH4Cl; (2) two bands (apparent molecular mass of 38 000 and 40 000) which cross-reacted with the antiserum to the Fe-protein, were found in nitrogen-fixing cells; (3) only one protein band, corresponding to the high molecular mass form of the Fe-protein, was found in cells grown with NaNO3 or low concentration of NH4Cl; (4) neither the Fe-protein nor the Mo-Fe protein was found in cells grown with urea; (5) the apparent molecular mass of the Fe-protein of Trichodesmium sp. NIBB1067 was about 5000 dalton higher than that of the heterocystous cyanobacterium, Anabaena cylindrica IAM-M1.

All reef-forming, or hermatypic, corals harbour photosynthetic endosymbiotic algae called zooxanthellae1–5, which are assumed to be predominantly a single dinoflagellate species, Gymnodinium microadriaticum Freudenthal6. The zooxan-thellae are essential for the well-being of their hosts7–9; nevertheless, little is known about how light affects the symbiotic association, especially regarding the numbers of zooxanthellae, their photosynthetic responses, and their overall productivity10–14. On the reefs of the Gulf of Eilat, Stylophora pistillata is an abundant hermatypic coral15; it is unique in that region in that it can adapt to a wide range of light intensities. In the high light intensities of lagoons or the upper areas of reefs, the corals are markedly lighter in colour than those living under ledges, in grottos, or near the reef floor (∼ 15 m; Fig. 1). We report here on the biochemical and physiological adaptations of S. pistillata to variations in light intensity spanning more than two orders of magnitude.

The cover picture represents a tree of the future, as described by Giacomo Ciamician in the first half of the 20th century when he wrote about “industrial colonies without smoke and without smokestacks” and “forests of glass tubes…︁and glass buildings…︁everywhere” inside of which “the photochemical processes that hitherto” had “been the guarded secret of plants” could take place. In their work on page 471,Michele Vittadello and co‐workers describe how photosystem II core complexes tethered to a gold surface may serve as efficient biomimetic photoelectron devices, bringing the realization of Ciamician's vision one step closer to reality. The cover image was prepared in collaboration with Nivi Alroy.

MICHAEL D. KING, CLAIRE L. PARKINSON, KIM C. PARTINGTON and ROBIN G. WILLIAMS (Eds.). 2007. Our Changing Planet. Cambridge University Press. ISBN 978‐0521828703, 400 p. US $45.00

In this study we use a model based on target theory to analyze steady-state photosynthesis-irradiance relationships in continuous light. From the average turnover time (τ) of photosynthetic units (PSUO2), numerical analyses of the model coefficients, and measurements of the light field and cell absorptivity, apparent absorption crosssections of photosystem II ($$\overline{\sigma }$$PSII) were determined for three species of marine unicellular algae grown at different irradiance levels. These cross-sections generally, but not always, increased with decreased growth irradiance. Additionally, the ratios of photosystem I/photosystem II reaction centers were calculated from measurements of oxygen flash yields and chlorophyll/P700 ratios. From the ratios of the reaction centers, cell absorptivity and the apparent absorption cross-section of photosystem II, the apparent absorption cross-sections of photosystem I ($$\overline{\sigma }$$PSI) were also calculated. Finally, on the basis of our calculated absorption cross-sections, we estimated the minimum quantum requirements for O2 evolution. Our results suggest that the absorption cross-sections of PS I and PS II vary independently and the minimum quantum requirements for O2 vary by more than twofold, increasing from 9.1 to 20.6 quanta/O2, as growth irradiance increases. The increase in quantum requirement corresponds to larger apparent cross-sections for photosystem I and higher carotenoid/chlorophyll ratios.